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I was at a well-known electric car company today and they claimed "instant torque." The quote was something like the following:

Driving this car is very different from a gas-powered vehicle. For example, it has instant torque when you press the accelerator. Unlike a gas vehicle where there is a slight delay.

My question is, where does this "instant torque" come from? Is the explanation as simple as the fact that electrons/electricity travel/s at the speed of light. But it takes time for an internal combustion engine to convert chemical potential energy into kinetic energy via thousands of tiny explosions? Or is there something else that could illuminate this explanation?

Edit: I just found this related question on a separate SE. Can anyone add more of a "fundamental physics explanation" to the set of answers?

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  • $\begingroup$ Throttle lag has several possible sources. Turbo spool-up is often the biggest one. Throttle induced down shifting and engine spool-up is a second. Torque converter delocking and spin-up is another. $\endgroup$ – Phil Sweet Nov 11 '17 at 3:15
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I think the instant torque claim mostly applies to "off the line" acceleration. That is from a standstill and electric motor has 100% of its available torque available at 0 rpm (mostly). The tradeoff is the an electric motor is always going to see a drop in torque with speed. Generally when you compare wheel torque availability as a function of speed between electric and a gas engine (with 5 forward gears) you have the following:

EC

Figure 1. The defining characteristic of an electric car is that peak torque is available at 0 rpm, in contrast to a gas engine which peak torque occurs at speed.

Also because of gearing, even though the torque and power values are much higher compared to gas engines, electric cars are limited to lower speeds because at some point the available torque goes to zero (even with ignoring air drag), compared to gas cars.

I think secondary is the fact that having only one gear torque is delivered to the wheels quicker with electric, compared to a gas car which takes much longer. Remember gasoline engines are practically an air pump (with fuel added later to match the air). The more air that can be pushed through the more efficient the engine is (and the more torque it makes). This means that it takes time to spool up the engine to the right rpm (usually requiring a downshift in gears) and for the inertia of the engine and the inertia of the air at the intake to be overcome. You might not realise it, but at higher rpm the air going through the intake ports approaches supersonic speeds around bends and curves. It takes a lot of energy to get air to those speeds and that energy is taken from the energy available for acceleration.

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    $\begingroup$ Nice diagram - source or is it yours? If it is, do you really expect constant acceleration in 5th? $\endgroup$ – Solar Mike Nov 11 '17 at 17:33
  • $\begingroup$ I drew it with PowerPoint. Many cars have a flat torque curve for drivability. Disregarding air resistance, yes most cars have flat acceleration over a broad range of speeds in the same gear. $\endgroup$ – ja72 Nov 11 '17 at 19:58
  • $\begingroup$ +1 for the diagram! The motor curve is well-known. And the gas curve is intuitive based on our everyday driving experience. But it is helpful to see them on paper. It really aids this discussion. $\endgroup$ – Mowzer Nov 11 '17 at 20:19
  • $\begingroup$ So, your diagram means that the car has the same rate of acceleration between 30 to 50 and 50 to 70 and 70 to 90 (assuming you are on a track and not breaking the speed limit...) - do real results ie measured times bear this out? $\endgroup$ – Solar Mike Nov 11 '17 at 20:37
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    $\begingroup$ Your electric torque curve is incorrect, or what you are showing is not relevant to the question. Over a large range, the torque of a electric motor is proportional to the electric current. What you are showing is the torque with a fixed voltage applied, but that's not how control systems for such applications work. The voltage required to get the same current goes up with speed. Eventually, the system can't produce enough power or voltage to maintain the current, but until then, you get all the torque you want. And you get it instantly, which is what the question is actually about. $\endgroup$ – Olin Lathrop Nov 12 '17 at 15:30
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ICEs (internal combustion engines) take some time to ramp up their output torque. Various mechanical systems have to react before more mixture is injected into cylinders, and that then makes more pressure upon combustion.

Think of a throttle-body engine as example. You push on the gas pedal, which opens the throttle valve more. That causes more air to rush into the intake manifold, which drags more gas with it due to the venturi in the carburetor. This lower pressure mixture gets sucked into a piston on the next intake stroke. Then there has to be a compression stroke before the mixture is ignited, and you finally get more torque as a result. Other things have to react to possibly change the timing, fuel/air ratio, etc.

Despite the above, the real perceived lag is due to the torque curve of the engine. Even though the engine is putting out more torque within a revolution or so, the engine's output is limited at low speed. Now matter how optimal the mixture being fed into the cylinders, it can't produce significantly more torque until it gets to a higher speed. That takes a while. Or, the transmission system has to shift to allow the engine a more optimum operating point to produce the additional output power. Either way, it takes time.

For electric motors, on the other hand, torque is proportional to current. This is independent of speed, so works just as well from a standing stop to cruising at highway speed. The current is controlled by producing pulses thru transistor that can do this 100s of 1000s of times per second. There is some lag to build up more current due to the inductance of the motor windings, but this lag is on the order of microseconds, milliseconds at most, and well below the human perception range. Remember that people have a hard time detecting a lag below about 50 ms. Having the motor be able to produce torque faster than humans can perceive the lag pretty much happens without any particular design effort to make it so.

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No, it's a characteristic that electric motors produce most, if not all, of their torque from zero rpm - which is why the electric cars are so good at getting off the line and how electric motors don't always need gearing to start heavy machinery - just lots of energy...

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The reason is inherent losses of the engine. Internal combustion engines consume a lot of torque they produce for own operation - they need to compress the fuel-air mix, they need to eject the combustion products, inject fuel-air mix, run the cooling system, run the fuel pump, run the alternator for providing electricity for spark plugs, overcome mechanical friction of the gearbox and so on. This all takes from the energy/torque produced through combustion and decompression, and the energy intake/output characteristics are quite non-linear; at low RPM there's really little usable surplus output torque and increasing it requires increasing RPM - the growth of surplus soon outpaces growth of demand, and the engine reaches peak power (...then the losses begin catching up again, and the engine reaches peak RPM).

It's also connected with the combustion process: through pressing the accelerator you modify the fuel to air ratio in the mix, making it more energetic, but you can only go so far extracting certain amount of energy from a single combustion (single piston cycle) - further increasing of amount of fuel will just lead to ejecting unburnt fuel or flooding the spark plug, instead of increasing the amount of energy produced. Instead, you're better off increasing the number of combustion events per second - the frequency of piston cycles - the RPM of the engine. That way instead of minuscule amount of very fuel rich mix producing modest amount of energy, you have a much large amount of moderately fuel-rich mix producing a lot of energy.

In electric motor there's both very little of such 'self-maintenance' losses and the amount of output torque doesn't depend on some kind of cycles, like frequency of combustion events - energy input can be ramped all the way up, no problems like flooding the spark plugs with too much of it.

Now this is the torque-RPM relation. The resulting time-torque relation is simple: as a combustion engine starts, its RPM are low, low torque output. If you want to increase it, you need to "spin it up", increase RPM - and that takes torque, which you don't have in abundance yet - so the process takes time. In electric engines, you get full output immediately.

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    $\begingroup$ This doesn't really answer the question. This gives reasons why electric motors can be more efficient than gasoline engines, but not why electric motors can produce more torque more quickly on demand. $\endgroup$ – Olin Lathrop Nov 10 '17 at 12:24
  • $\begingroup$ @OlinLathrop: ...um, I detailed the correlation between power output and RPM for combustion engines. I thought the concept that motor's RPM at start is zero and must be ramped up which takes some time is pretty obvious? $\endgroup$ – SF. Nov 10 '17 at 12:51
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Instant torque means that you have have <100 milliseconds delay from pedal/lever/throttle movement to maximum corresponding acceleration of the vehicle, which is controlled by a phase controller that sends pulses to the motor. The controller and motor are at most 100 ms time response, at least 10 ms.

The clutch prevents their being a direct connection in between the motor and the wheels. It also prevents continuous acceleration from low to high gear. From standstill, the delay for clutch engagement is about 1-3 seconds, compared to 50-100 milliseconds on EV's.

An electric car has zero rotation difference in between the motor axle and the wheel rotation.

The motor-coil-pulses apply force to magnets, that's why you need a slightly slow controller to redistribute DC battery energy into multiple motor "phase" wires.

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